The present invention relates to a pattern forming method for forming fine patterns with high accuracy and throughput. More particularly, the present invention relates to a method for generating pattern data for light exposure and charged particle beam exposure from device design patterns in order to transfer patterns on the same photosensitive material using light exposure and charged particle beam exposure.
Photolithography in semiconductor manufacturing steps is widely used for the production of devices thanking to its advantages such as simplicity of processing and low cost. Recently, elements as fine as 0.25 .mu.m or less have been realized as a result of the introduction of short wavelength utilizing KrF excimer laser light sources. In order to improve fineness further, efforts are being put on the development of ArF excimer laser light sources having shorter wavelengths and Revenson type phase shift masks which are regarded promising as mass production lithographic tools compatible with 0.15 .mu.m rules. However, there are many disadvantages to be solved to realize them, which has increased the time required for the development of the same and behind the rate at which devices are made finer and it is being worried about being impossible to catch up a fineness speed of the device.
On the contrary, electron beam lithography which is the most promising candidate as the post-photolithography technique (in the following description, "electron beam lithography" obviously implies charged particle lithography) has proved itself capable of processing on the order of 0.01 .mu.m using narrowed beams. Although this technique has no pressing disadvantage from the viewpoint of improving fineness, it has a disadvantage with throughput when viewed as a tool for the mass production of devices. Specifically, this technique inevitably takes time because it sequentially draws fine patterns one by one. In order to reduce such drawing time, several apparatuses have been developed which employ methods such as exposure method of character projection wherein repetitive parts of an ULSI pattern are simultaneously drawn partially. However, the use of such apparatuses has been still unsuccessful in catching up the through put of photolithography.
As a method of increasing the throughput of electron beam lithography, proposals have been made on the so-called mixing and matching of light and electron beams in the same layer. This method is to perform pattern transfer on to the same resist using light exposure and electron beam exposure to reduce areas subjected to electron beam exposure, thereby increasing the number of wafers which can be processed by an electron beam lithography apparatus per hour. For example, Jpn. Pat. Appln. KOKAI Publication No. 4-155812 discloses the use of light exposure and electron beam exposure employing a phase shift mask in transferring patterns on to the same resist during a lithographic step for pattern formation. According to the publication, most of patterns to form an element are transferred using the phase shift mask and areas having disadvantages attributable to the position of the phase shifter are corrected by means of electron beam lithography, thereby reducing areas to be subjected to electron beam lithography as much as possible to increase the number of wafers which can be processed by the electron beam lithography apparatus per hour.
Although this method reduces areas to be drawn using electron beams, it can not be adapted to finer devices in future because it is not capable of pattern transfer at resolutions below the limit of the resolution of the phase shift mask. Especially, ultra-resolution techniques such as the Revenson type phase shift mask may be limited to patterns having regular lines and spaces of memory LSIs and can not be adapted to increased fineness of random patterns which are characteristic of logic devices.
Further, since LSIs are manufactured through many repetitive lithographic steps, they have variations in processing at those steps and errors in alignment of layers. It is impossible to eliminate those factors completely, although various measures are being taken to suppress them as much as possible. This disadvantage is encountered also in performing pattern transfer on to the same resist using light exposure and electron beam exposure. According to the above-described method, pattern sizes become too large or small or gaps are formed at areas where patterns formed by light exposure and electron beam exposure are connected because such variations in processing and alignment errors are not taken into account in this method.
The variable beam shaping method capable of providing a variety of beam shapes is used to form irregular patterns as seen on, for example, logic circuits using electron beam exposure. The variable beam shaping method controls an optical overlap between a first shaping aperture and a second shaping aperture using a shaping deflector, so that the shapes and sizes of electron beams can be varied with flexibility.
However, a disadvantage arises in that the accuracy of beam sizes is directly affected by the deflecting accuracy of the shaping deflector. Further, insulation materials are gradually deposited on the surface of structures between the first and second shaping apertures and are charged to cause a change in an electrostatic field in the path of electron beams, which results in a shift of the position of an image of the first aperture projected upon the second shaping aperture from a desired position. Since the amount of the deposited insulation materials and the charge thereof change over time, beam sizes also change over time. Therefore, even if the beam sizes are calibrated before lithography, patterns formed at the beginning and the end of lithography are in different sizes because the accuracy of beam sizes gradually changes during lithography.
The disadvantage of overlapping deviations will now be described with reference to FIGS. 1A through 1D. If a design pattern is divided into a light-exposed pattern A and an electron-beam-exposed pattern B simply based on the sizes thereof, since the light-exposed pattern A and electron-beam-exposed pattern B are contact to each other (FIG. 1A), an error in the overlapping of the light exposure and electron beam exposure as described above results in a gap in the area which must be contact (FIG. 1B) to disable normal operation of the element.
An article titled "Electron beam/DUV intra-level mix-and-match lithography for random logic 0.25 .mu.m CMOS" (R. Jonckheere et al., Microelectronic Engineering 27 (1995) pp. 231-234) discloses mix-and-match in the same layer wherein pattern transfers on the same resist are performed using light exposure utilizing a deep-UV stepper and electron beam exposure utilizing a Gausian electron beam lithography apparatus at a lithographic step for pattern formation. According to this article, patterns are divided using 0.4 .mu.m as a reference, and patterns of 0.4 .mu.m or more are exposed by deep-UV light whereas patterns less than 0.4 .mu.m are drawn directly on a wafer using electron beams. In order to absorb errors in overlapping the light exposure and electron beam exposure, the light-exposed patterns and electron-beam-exposed patterns are exposed such that they overlap by 0.1 .mu.m (FIG. 1C). By overlapping the patterns in such a manner, it is possible to prevent discontinuation of patterns in areas where they are to be contact even if there is overlapping deviations (FIG. 1D).
The above-cited article discloses two methods for generating such overlapping patterns. The first method is to move the outline line of a light-exposed pattern outward of the pattern to increase the sizes of the pattern, thereby overlapping it with an electron-beam-exposed pattern (such a processing is referred to as "sizing processing" or "resize processing"). The second method is to move the outline line of an electron-beam-exposed pattern outward of the pattern to increase the sizes of the pattern, thereby overlapping it with a light-exposed pattern. According to the first method, however, an increase in the sizes of a light-exposed pattern decreases distances between patterns in violation of design rules. According to the second method, the sizes of patterns are increased by the overlapped pattern, which disallows a desired pattern to be obtained.
The fabrication of masks to manufacture many types of elements in small quantities takes time. As means for solving this disadvantage, Jpn. Pat. Appln. KOKAI Publication No. 1-293616 discloses a processing wherein a group of functional blocks common to various semiconductor elements on the same resist are exposed by light and a method wherein patterns unique to each semiconductor element are drawn using electron beams. Specifically, a mask for the areas common to various elements is prepared in advance, and electron beam lithography is used in the remaining areas having different patterns. This method makes it possible to shorten the period spent from the designing of elements until the manufacture of the same because there is no need for preparing a mask for each type of element. However, this method is similar to the above-described method in that it can not be used where there are patterns in functional blocks that require resolutions below the limit of resolution of light exposure. Further, patterns to be drawn using electron beams are wiring areas and the like, and the use of electron beam exposure inevitably takes time because patterns are sequentially drawn one at a time. As a result, this method is difficult to use in lithography systems for drawing fine patterns at a high speed.
As described above, the mix-and-match of light and electron beams in the same layer which has been performed to improve throughput has had disadvantages in that the resolving power of electron beam exposure is not fully utilized and in that throughput is not improved to the same level as that achievable with an optical stepper.
In order to solve the above-described disadvantages, the inventors have already proposed a lithography system for exposing the same light-sensitive material to light and electron beams which provides both of excellent resolving power beyond that of light provided by electron beam exposure and throughput equivalent to that achieved by an optical stepper.
As described above, the conventional mix-and-match of light and electron beams in the same layer has the disadvantages listed below.
(1) It does not fully utilizes high resolving power and high throughput characteristic of electron beam exposure and light exposure, respectively. PA1 (2) An error in the overlapping of light exposure and electron beam exposure can reduce pattern accuracy in areas where patterns exposed by light and electron beams are contact to each other. An effort to avoid this will make it impossible to obtain a desired pattern size. PA1 (3) While the variable beam shaping method is used to irregular patterns as seen on logic circuits utilizing electron beam exposure, pattern accuracy is reduced in this case due to changes in the accuracy of beam sizes over time. PA1 (4) No solution has been found to avoid the influence of electrons that are scattered backward. PA1 (1) To provide a method for lithography having high resolution and throughput capable of achieving high pattern accuracy. PA1 (2) To provide a method for generating pattern data capable of preventing reduction in pattern forming accuracy attributable to misalignment between light exposure and electron beam exposure at areas where various patterns are connected without oversizing patterns exposed by light and patterns exposed by electron beams and to realize a lithography system which has excellent resolving power beyond that of light provided by electron beam exposure, high pattern forming accuracy and throughput equivalent to that achievable with an optical stepper and which can be adapted to mass production that follows photo-lithography. PA1 (3) To provide a method for pattern formation which compensates for the effect of back-scattered electrons that fall on patterns exposed by light in the vicinity of areas exposed by electron beams during electron beam exposure and which has preferable pattern resolution, controllability of sizes, excellent resolving power beyond that of light provided by electron beam exposure and throughput equivalent to that achievable with an optical stepper. PA1 (1) A first outline movement and a second outline movement have the same absolute value. PA1 (2) The first outline movement and second outline movement have different absolute values. PA1 (3) The first and second outline movements are determined in consideration to errors in the alignment of light exposure and electron beam exposure on the same photosensitive material, the accuracy of the electron beam size from an electron beam lithography apparatus, value of the pattern size change in resist processing and value of the pattern size change in etching. PA1 (4) The extraction of patterns exposed by electron beams is carried out by extracting patterns having sizes smaller than a reference size. PA1 (5) A boundary size between light exposure and electron beam exposure is defined; a value obtained by correcting the boundary size by the first outline movement is used as a reference value; and patterns smaller than the reference value are extracted as patterns exposed by electron beams. PA1 (6) Prior to a step of converting patterns exposed by electron beams into a data format for an electron beam lithography apparatus, a step is performed to move the positions of at least either of shorter or longer sides of extracted patterns exposed by electron beams in a direction perpendicular thereto. PA1 (7) Prior to a step of converting patterns exposed by electron beams into a data format for an electron beam lithography apparatus, a step is performed to move the positions of sides of extracted patterns exposed by electron beams which are in contact with patterns exposed by light in a direction perpendicular thereto. PA1 (1) First, second and third outline movements have the same absolute value. PA1 (2) The first, second and third outline movements have different absolute values. PA1 (3) The first, second and third outline movements are determined in consideration to errors in the alignment of light exposure and electron beam exposure on the same photosensitive material, the accuracy of the electron beam size from an electron beam lithography apparatus, value of the pattern size change in resist processing and value of the pattern size change in etching. PA1 (4) A boundary size between light exposure and electron beam exposure is defined. Patterns smaller than the boundary size are classified as patterns exposed by electron beam exposure, and patterns greater than the boundary size are classified as patterns exposed by light. PA1 (5) A boundary size between light exposure and electron beam exposure is defined; a value obtained by correcting the boundary size by the first outline movement is used as a reference value; patterns greater than the reference size are classified as patterns exposed by light; and patterns smaller than the reference value are classified as patterns exposed by electron beams. PA1 (6) Prior to a step of converting patterns exposed by electron beams into a data format for an electron beam lithography apparatus, a step is performed to move the positions of at least either of shorter or longer sides of separated patterns exposed by electron beams in a direction perpendicular thereto. PA1 (7) Prior to a step of converting patterns exposed by electron beams into a data format for an electron beam lithography apparatus, a step is performed to move the positions of sides of separated patterns exposed by electron beams which are in contact with patterns exposed by light in a direction perpendicular thereto. PA1 (1) The predetermined correction is performed using photomasks used for light exposure in A1 (A2). PA1 (2) The predetermined correction is performed by varying the pattern sizes of the photomasks used for light exposure in A1 (A3). PA1 (3) The photomasks are generated by a charged particle beam exposure apparatus employing proximity effect correction for dose control and wherein the predetermined correction is carried out by modulating a map of incident dose used during the generation of the photomasks in A1 through A3 (A4). PA1 (4) The modulation of the map of incident dose is performed depending on whether there is any effect of back-scattered charged particles at the time of the exposure of charged particle beams in A4 (A5). PA1 (5) The modulation of the map of incident dose used in photomask making is carried out by obtaining the incident charged particles from a relational expression: pattern size change per charged particle on wafer.times.back-scattered charged particles=reduction ratio of stepper.times.pattern size change per charged particle on photomask.times.incident charged particles in A4 (A6). PA1 (6) The process of generating data for the map of incident dose comprises the step of: obtaining a map of proper doses for drawing a pattern exposed by light on a photomask substrate; obtaining a distribution map of energy accumulated by back-scattered charged particles during the drawing a pattern exposed by charged particles on a wafer; converting the distribution map of energy accumulated by back-scattered charged particles into a distribution map of variation of the pattern size exposed by light; enlarging the distribution map of variation of the pattern size exposed by light by an inverse multiple of the reduction factor of a stepper for light exposure; converting the enlarged distribution of variation of the pattern size exposed by light into a distribution map of converted values of photomask exposure doses; and subtracting the distribution map of converted values of photomask exposure doses from the map of proper doses in A4 (A7). PA1 (7) The predetermined correction is carried out by modulating the pattern size exposed on photomasks during the fabrication of the same in A1 through A3 (A8). PA1 (8) The modulation of the pattern size is carried out depending on whether there is any effect of back-scattered charged particles during the exposure of charged article beams in A8 (A9). PA1 (9) The modulation of the pattern size is carried out by obtaining the amount of the pattern size change on photomask from a relational expression: ratio of pattern size change on wafer to back-scattered charged particle.times.back-scattered charged particles=reduction ratio of stepper.times.pattern size change on photomask in A8 (A10). PA1 (10) The step of generating data for the map of pattern size comprises the steps of: obtaining a distribution map of energy accumulated by back-scattered charged particles during the drawing a pattern exposed by charged particle beams on a wafer; converting the distribution map of energy accumulated by back-scattered charged particles into a distribution map of variation of the pattern size exposed by light; enlarging the distribution map of variation of the pattern size exposed by light by an inverse multiple of the reduction factor of a stepper for light exposure; and subtracting the enlarged distribution map of variation of the pattern size exposed by light from the pattern exposed by light drawn on the photomask substrate in A8 (A11). PA1 (11) The predetermined correction is carried out by correcting device pattern data in A1 (A12). PA1 (12) The predetermined correction is carried out by keeping a definite distance between the area of the pattern exposed by light and the area of the pattern exposed by charged particle beams except in a pattern area connecting those areas in A1 or A12 (A13). PA1 (13) The constant distance is a distance corresponding to the scattering diameter of back-scattered charged particles during the exposure of charged particle beams in A13 (A14). PA1 (14) The constant distance is determined by the degree of the effect of back-scattered charged particles during the exposure of charged particle beams in A13 (A15). PA1 (15) The predetermined correction is carried out by varying the sizes of the pattern of a photomask used for light exposure and wherein the sizes of the pattern of the photomask are varied by varying the dose based on the relationship between the variation of the dose of charged particle beams used for drawing the photomask and the variation of the pattern sizes identified in advance in A1 (B1).